High frequency vacuum tube circuit



Feb. 21, 1956 Filed Sept. 17 1945 R. C. PECK HIGH FREQUENCY VACUUM TUBECIRCUIT 2 Sheets-Sheet l I; E; J-

6 OU T PUT po 5 62 AAAA gwuem to'b RICHARD C. PECK @QQM W Feb. 21, 1956R. c. PECK 2,735,941

HIGH FREQUENCY VACUUM TUBE CIRCUIT Filed Sept. 17, 1945 2 Sheets-Sheet 2TTEE E l sz awe/whom RICHARD C. PECK WWLW 2,135,941 men FREQUENCY VACUUMTUBE CIRCUIT 7 Richard C; Peck, Washington, D. C. indication September17,1945, Serial N0. 616,948

. 8 Claims. 01. 250-36 '(drantdimiaer Title 35, U. s. C0118 1952 See.266) invention relates to vacuum tube amplifying circans; moreparticularly it relates to a means for employing s viitibnal vacuumtubes as oscillators or amplivery" highfreqiiencies.

his so me of flight is the factor which 7 governs the pper limit offrequency at which a convencuum tube may be usefully employed. When e syis raised to such a high value that the time t eraser-r6115 passingbetween the electrodes is an able fraction of a period, conventionalvacuum tubes and conventional circuits cease to function.

As a practical matter, however, the physical size of vacuum tube and thedistributed capacitance and 'iiiducffarice resulting therefrom have inthe past fixed a i ifictic'al upper limit of frequency much lower thanthe or an liniitimposed by transit time of electrons. ect of thisinvention is to provide a means for omlhgthe frequency-limiting effectof distributed ie'icta cs iii conventional vacuum tubes and thereby i6 6possible useful employment of conventional t frequencies closelyapproaching the theoretical transit-time limit.

The invention will be discussed and described with c rice to theappended drawings, of whichi i 'gure 1 is diagrammatic representation ofa highfrequency oscillator employing a conventional vacuum tube in aColpitts circuit; this figure being used to exth' principle of operationof the invention;

Figure 2 is'a diagrammatic showing of a high-frequency oscillatoremploying an embodiment of the presefht invention in conjunction with aconventional vacult Figure 3 is a diagrammatic showing of anotheremhddim tit of the invention wherein conventional vacu- 'ss are employedas a frequency multiplying ampli er e 4 is a drawing which showsdiagrammatically an od nt ofthe invention employing a split-anodenegative resistance magnetron as a high frequency oscila i nre 5 is aview,.partly in cross section, of a physical ctidrl or; high frequencycomponents for use in theembodiment of Figure 3; and

I Figure 6 is another cross section view of the apparatnc or Figure 5.

Referring to Figure 1, the oscillator therein shown eomprisesa triodevacuum tube 5, connected in a form the (Eolpitts circuit adapted forultra-high-frequennice-. "Transmission line 6 serves as tank circuit forthe oscillator; it consists of two conductors, numbered 61 and '62respectively. One terminal of conductor 61 is connected to the plate oftube 5; the other terminal of conductor 61 is connected through R. F. ofchoke coil 6 to the positive side of D.-C. source 10. One terminal 6?"conductor 62 is connected to the grid of tube 5; the other. terminal ofconductor 62 is connected to ground tn gridleak resistor 9. Conductors61 and 62 are i6 by condenser 7 at the terminals adjacent choke 8 to orCondenser 7 has substantially zero im- 'at the operating frequency. Line6 is, there- States Patent O 2,735,941 Patented Feb. 21, 1956 fore, forhigh frequency currents, a short-circuited section having its openterminals connected between grid and plate of tube 5. The cathode oftube 5 and the negative side of source 10 are grounded. R. F. power maybe taken from the oscillator in any manner appropriate to the nature andimpedance of the load; in this embodiment the R. F. output terminals 4are connected respectively to ground and to the plate of tube 5 throughblocking condenser 3.

The effective length of transmission line 6 includes the length ofconductors 61 and 62 plus the length of the leads to and within tube 5and the actual length of the tube electrodes themselves. According totransmission line theory such a line, open at one end andshort-circuited at the other, is capable of oscillating in an infinitenumber of modes. Of these the fundamental mode is that wherein the totaleffective length of the line equals one quarter wavelength. The nexthigher mode has a frequency of oscillation three titmes as great as thefundamental frequency; in that mode the effective length of the lineequals three quarters of a wavelength.

It is a property of self-excited vacuum tube oscillators thatoscillation will be maintained at that frequency,

wherein losses are lowest for the particular oscillatory circuit beingemployed. In conventional oscillators using transmission line tankcircuits the frequency adopted is that of the fundamental mode of theline. Consequently, while a line-controlled vacuum tube oscillator istheoretically capable of oscillating in many modes, a particulartransmission line tank circuit, employed with a particular tube in aconventional circuit, does by its constants determine a unique frequencyof oscillation.

In the light of the foregoing paragraphs, assume that a higher frequencyis desired from the vacuum tube oscillator of Figure 1. The obviousprocedure to ef fect an increase in frequency is to shorten line 6.- Asline 6 is made shorter and shorter, the frequency of oscillation growshigher and higher until at last line 6 is eliminated entirely andcondenser 7 is shortened directly between the grid and plate terminalsof tube 5. The frequency of oscillation obtained under these conditionsis the highest obtainable from the tube by conventional means. Moreoverthe efficiency and stability become very poor as the external lineshrinks-in length, and in practice it is often impossible to obtain anyoscillation at all when the external line has been reduced to thevanishing point. In any case the maximum frequency obtaintable is wellbelow the theoretical limit imposed by transit time considerations.

With respect to the embodiments of the invention herein described, byenclosing a transmission line tank circuit in an appropriately designedresonant cavity, oscillation at the fundamental mode can be preventedand the tube can be forced to set up and maintain oscillations in thetransmission-line tank circuit at one of the lines higher modes. Thusthe output frequency of the oscillator may be made an odd multiple ofthe resonant frequency of the line, and tubes may be employed asoscillators virtually up to the frequency limit imposed by transit time.This result can be obtained because a resonant cavity short-circuits andsuppresses all electromagnetic fields within it having a frequency lowerthan a critical cut-off frequency while allowing electromagnetic fieldshaving a frequency near the cavitys resonant frequency to be maintainedwithin the cavity.

The details of design may be best explained with reference to Figure 2,which shows, in diagrammatic form, one embodiment of the invention. InFigure 2, an oscillator is shown, incorporating tube 15. The tankcircuit connected to tube 15 consists of transmission line 16, to getherwith the leads and tube elements associated therewith. Line 16 comprisesparallel conductors 161 and 162. One end of conductor 161 is connectedto the plate of tube the other end is connected through R. F. choke coil18 to the positive side of D.-C. source 20. One end of conductor 162 isconnected to the grid of tube 15, the other end is connected throughgrid leak resistor 19 to ground. The ends of conductors 161 and 162adjacent choke 18 and resistor 19 are joined by condenser 17, which is asubstantially zero impedance at the operating frequency. The cathode oftube 15 and the negative side of source 20 are grounded.

Entirely surrounding line 16 and a portion of the envelope of tube 15 isresonant cavity 13. Cavity 13 may be cylindrical in shape or it may havea rectangular or other cross-section. Its size in this embodiment issuch as to cause the oscillator to resonate at a frequency about threetimes the natural frequency of the tank circuit comprising line 16 andthe leads and tube elements associated therewith. Cavity 13 is closedexcept for an aperture in one end adequate to admit a portion of theenvelope of tube 15 and two small apertures 11 in the opposite end,necessary to allow the leads from line 16 to be brought out. R. F.output may be taken in any manner appropriate to the impedance of theload; in Figure 2 the R. F. output terminals 14 are connected betweenground and the plate of tube 15. D. C. voltage on the plate is blockedfrom the R. F. load by condenser 21.

Cavity 13 has a physical dimension of approximately three-quarterwavelength at the resonant frequency of the system, but the capacitiveloading of line 16 on the cavity 13 decreases its electrical length to ahalf wavelength. Since cavity 13 is essentially closed at both ends thelowest mode it can sustain is the half wavelength mode, which means line16, connected for odd quarter wavelength operation, is forced into thethree-quarter wavelength mode or higher.

When cavity 13 is oscillating in its fundamental mode the wavelength ofoscillation is about twice the length of the cavity, a conditionrequiring substantially zero electric field at each end of the cavityand maximum electric field at a point about midway between the ends.This set of conditions conforms very well to three-quarter waveoscillation by the tank circuit associated with tube 15, since suchoscillation places an electric-field node at the shorted end of line 16,an electric field maximum a quarter wavelength from the end, or aboutmirway between the ends of cavity 13, and another electric field node atthe point one-half wavelength from the shorted end. The actual tubeelements and the portion of the leads within the tube envelope externalto cavity 13 constitute, electrically, a third quarter-wavelength, whichplaces an electric field maximum between the plate and grid. (Theelectrical quarter-wave Within the envelope of tube 15 will normally bemuch less than a physical quarter-wave in length, because of the loadingeffect of the plate-to-grid capacitance.)

Whereas the cavity 13 encourages oscillation in the three-quarterWavelength or triple frequency mode, it makes oscillation of tube 15 inthe tank circuits fundamental mode impossible, because the frequency offundamental mode oscillations is below the cutoff frequency of thecavity and cannot be sustained therein.

By employment of the present invention the upper frequency limit ofstandard tubes can be greatly increased over the frequencies obtainablewith conventional circuits. For example, an RCA 8012 tube, whose upperfrequency limit in conventional circuits is about 600 mc./s, has beenmade to oscillate, by use of this invention, at frequencies above 800rnc./s.

Moreover, because of the high quality factor of the cavity, plus thehigher quality factor obtainable in the tank circuit when its lengthneed not be of vanishing proportions, improved etiiciency and frequencystability are obtained at very high frequencies with this invention.

This invention may also be applied to a frequency mul- 4 tiplyingamplifier. Figure 3 shows a typical embodiment of this sort wherein apairt of tubes are employed in pushpull as a frequency tripler.Referring to Figure 3, transmission line tank circuit 23, approximatelyone-quarter Wavelength long for the frequency of the input signal,consists of parallel conductors short circuited at one end. The openends of line 23 are connected respectively to the grids of tubes 25 and35, the short circuited end of line 23 is connected to ground throughcondenser 28 and resistor 29 in parallel. R. F. energy at the inputfrequency may be applied to input terminals 30. Coupling loop 22,connected to terminals 30, is inductively coupled to line 23. Thecathodes of tubes 25 and 35 are grounded. Transmission line tank circuit36 consists of two parallel conductors short circuited at one end. Theopen ends of line 36 are connected respectively to'the plates of tubes25 and 35. The physical dimensions. of line 36 are approximately similarto those of line 23 In this embodiment the cavity does not whollyenclose the transmission line and part of the tube envelope as in theembodiment of Figure 2. If the frequency sought is not so high as toapproach the absolute maximum, the cavity may be placed around only thetransmission line itself as in Figure 3. Cavity 33 in Figure 3resonates, with line 36 within it, at three times the signal frequencyapplied to input terminals 30. Cavity is closed but for apertures 32 inone end, to permit the passage of the conductors of line 36, and smallaperture 38 in the opposite end. The short circuited end of line 36 isconnected to the positive side of D.-C. source 40 by a wire passing outof cavity 33 through aperture 38. Condenser 37 is connected between theshort-circuited end of line 36 and the adjacent end of cavity 33. Theoutside surflaee of cavity 33 and the negative side of source 40in;grounded. R. F. output from this circuit, as with the previousembodiment shown, may be taken in any man ner appropriate to theimpedance of the load. In Figure 3 output terminals 27 are connected tothe respective plates of tubes 25 and 35 through blocking condensers 26.The output voltage at terminals 27 is balanced rela, tive to ground. I

The operation of the embodiment of Figure 3 is similar in principle tothat of the oscillator shown in Figure 2. The plate tank circuit isprevented by cavity 33 from oscillating at its normal fundamentalfrequency and is consequently set into oscillation in itstriple-frequency, or three-quarter wavelength, mode by the energysupplied by tubes 25 and 35. The output power taken from ter minals 27has, therefore, three times the frequency'of the energy fed to inputterminals 30. By employing the principles of this invention,conventional tubes may be made to operate efficiently and stably atfrequencies well above the practical frequency limits of conventionalfre quency multiplier circuits. I

Figure 4 shows an embodiment of the invention efii ploying a split-anodenegative resistance magnetron'as a high-frequency oscillator. Theemployment of the invention in this manner permits realization of higherfrequency output from the magnetron than is available by conventionalmeans and yields a higher degree of frequency stability than is usuallyobtainable from negative resistance magnetrons. Referring to Figure 4;tube 45 is a magnetron tube of the negative resistance split anode type.It is placed in a steady magnetic field provided by" a magnet which isnot shown in the drawing. The two anode segments of the magnetron 45 areconnected re-' spectively to the open terminals of a short-circuitedtransmission line section 4-6, which serves as resonant circuit for themagnetron oscillator. Line 46 is housed in a closed conducting resonantcavity 43, the conductors of line 46 protruding from the closed cavity43 through apertures 53. The short-circuited end 47 of line 46 isconnected to the end of cavity 43 at point 48, situated at a pointwithin cavity 43 whereat the electric field is minimum when the cavityis oscillating in its fundamental mode. The positive side of D.-C.source 50 is connected to the outer surface of cavity 43 and thenegative side of source 50 is connected to the cathode of magnetron tube45. R. F. output may be taken from the oscillator by connecting a loadto output terminals 51, which are connected to the ends of coupling loop52. Coupling loop 52 extends through small apertures 54 in cavity 43 andis inductively coupled to line 46.

In operation, the properties of the magnetron cause a negative A.-C.resistance to appear across the anode segments of tube 45, supplyingenergy to transmission line tank circuit 46 and sustaining oscillationstherein. As in the previously described embodiments, cavity 43 preventsthe line 46 from oscillating in its fundamental mode. Line 46 can anddoes oscillate in its three-quarter wavelength mode, however, sinceoscillation in that mode produces fields which coincide with andreinforce oscillations in the cavity. The net effect, as in the otherembodiments of the invention herein shown, is to permit successful andstable operation at frequencies closely approaching the limit imposed bytransit time. In an application where the frequency required is verynear the limit obtainable, the cavity may be constructed as in Figure 2,to Wholly enclose line 46 and part of the envelope of magnetron 45 aswell, thus reducing the portion of the tank circuit outside the cavityto the anode segments themselves and a part of the leads within theevacuated tube envelope.

Figures and 6 illustrate typical physical construction which might beemployed for the cavity and transmission line assembly used in theinvention. Figure 5 is a view, partly in cross section, of an assemblysuitable for use in the embodiment of Figure 3. The section in Figure 5is taken along the axis of the resonant cavity. Figure 6 is a view ofthe same embodiment taken in section in a plane perpendicular to thecavity axis as indicated by the dotted line 6-6 in Figure 5.

Referring to Figure 5, the resonant cavity in this representativeembodiment is cylindrical; the principal element 73 of the cavity istubular and formed of rigid conducting material. The remainder of thecavity is formed by circular end plates 72 and 74, also formed of rigidconducting material. End plate 72 is secured to element 73 by aplurality of screws 83, symmetrically disposed around the periphery ofplate 72 and threaded into holes drilled and threaded therefor inelement 73. End plate 74 is similarly secured to the opposite end ofelement 73 by screws 81. Transmission line 76 consists of parallelconductors joined at one end, forming a bifurcated conducting member asshown. The open ends of line 76 pass through threaded insulatingbushings 82, which are securely threaded into apertures in end plate 72.The axial openings in bushings 82 through which the conductors of line76 pass are proportioned to effect a snug fit therefor.

From the closed or short-circuited end of line 76 a stub extension 78protrudes along the axis of line 76, passing through bushings 77 and 87which are fitted into an aperture in the center of end plate 74. Aflange 84 on stub '78 fits snugly against bushing 77; the end of stub 78is threaded and nut 79 is tightly threaded thereon so as to holdbushings 87 in place and effect rigid mechanical support for line 76.External connection to the short circuited end of line 76 may beeffected by connection to the external end of stub 78. Stub 78 and thesurrounding conducting surface of end plate 74 form the condenser shownin diagrammatic form in Figure 3 as condenser 37.

Figure 6 shows a section of the apparatus of Figure 5 taken in a planeperpendicular to the section in Figure 5. The section shown in Figure 6is indicated on Figure 5 by the dotted line 66, looking toward end plate72 as indicated by the arrows in Figure 5. Tubular element 73 is seen incross section, as are the conductors of line 76. End plate 72 andinsulating bushings 82 appear in plan view as shown in the drawing.

It will be understood that the embodiments of the invention shown anddescribed herein are exemplary only, and

that the scope of the invention is to be determined by reference to theappended claims.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

What is claimed is:

' 1. A distributed-constants tanks circuit enclosed at least in part bya cavity resonator and electrically coupled thereto, said cavityresonator having a cutoff frequency higher than the frequency of thefundamental mode of oscillation of the tank circuit.

2. A tank circuit comprising a section of transmission line, said tankcircuit enclosed at least in part by a cavity resonator and electricallycoupled thereto, the lowest frequency of resonance of said cavityresonator being approximately an integral multiple of the frequency ofthe fundamental mode of oscillation of the tank circuit.

3. The combination of an oscillator comprising a distributed constantstank circuit having a given fundamental frequency of resonance, and acavity resonator having a higher fundamental frequency of resonance thansaid tank circuit enclosing at least in part said tank circuit andelectrically coupled thereto, said cavity resonator acting tooperatively suppress fundamental mode oscillations in said tank circuit.

4. The combination of an oscillator comprising a tank circuit having asection of transmission line, said tank circuit having a givenfundamental frequency of resonance, and a cavity resonator having ahigher fundamental frequency of resonance than said tank circuitenclosing at least in part said transmission line and electricallycoupled thereto, said cavity resonator serving to operatively suppressfundamental mode oscillations in said tank circuit.

5. An oscillator comprising a vacuum tube having at least first andsecond electrodes; a tank circuit effectively comprising the first andsecond electrodes, leads thereto within the tube, and a section oftransmission line; and a cavity resonator having a cutoff frequencyhigher than the frequency of the fundamental mode of oscillation of thetank circuit, said cavity resonator enclosing the section oftransmission line and enclosing at least in part said leads within thetube, said cavity resonator serving to suppress fundamental-modeoscillations in the tank circuit.

6. An oscillator comprising a vacuum tube having a grid and a plate; atank circuit effectively comprising the grid, the plate, leads theretowithin the tube, and a section of transmission line; and a cavityresonator having a cutoff frequency higher than the frequency of thefundamental mode of oscillation of the tank circuit, said cavityresonator enclosing the section of transmission line and enclosing atleast in part said leads within the tube, said cavity resonator servingto suppress fundamental mode oscillations in the tank circuit.

7. An oscillator comprising a magnetron tube, a distributed constantstank circuit operatively coupled to said magnetron tube, and a cavityresonator enclosing the tank circuit at least in part and of suchdimensions as to operatively suppress fundamental-mode oscillations inthe tank circuit.

8. An amplifier having input means for applying high frequency energy tothe input means, a vacuum tube fed by the input means; a tank circuithaving distributed constants; a cavity resonator enclosing the tankcircuit at least in part and of such dimensions as to operativelysuppress fundamental mode oscillations in the tank circuit, and couplingmeans operative to feed energy from the vacuum tube to the tank circuit.

References Cited in the file of this patent

